In the theoretical world of chords, the dominant seventh chord is unique. Here are the notes of a D7 chord:
D – (F#) – A – (C )
The notes that define the core function of a dominant seventh chord are the 3rd and the 7th notes called a “tritone.” The two notes are the 3rd (F#) and the 7th (C). If you count the frets between (F#) and (C) there are 6 frets. The name “tritone” means a “tone” equals 2 frets and “tri” means 2×3 or six frets total. The remaining tones of the D7 chord, D and A are theoretically optional.
As an illustration, here’s a video showing a quick and easy finger picking blues in G. The progression is the standard G7 – C7 – D7 chords using just the two note tritones with the open banjo strings. The G7 tritone notes are located on the 2nd string 6th fret and the 3rd string 4th fret. Move the two notes down one fret for C7 and up one fret for D7:
G7 = G – (B) – D – (F)
C7 = C – (E) – G – (Bb)
D7 = D – (F#) – A – (C)
Here are the notes of a D7 chord with an added 9th:
D7(9) = D – F# – A – C – E
Below are four different chord positions with the same notes. They all sound like D7. Notice that there are no “D” notes in any of the forms. Because each chord position has the F# and C tritones, the root note D is not necessary. An added E note or 9th enhances and extends the D7 sound:
1st form: F#m7b5 = (F#) – A – (C) – E
2nd form: Am6 = A – (C) – E – (F#)
3rd form: D9 (no root) = (C) – E – (F#) – A
4th form: D9 (no root) = E – (F#) – A –(C)
What’s interesting about these chords is that they all sound like a D7th chord but contain no “D” note.
Learning to view the neck in partial chord fragments sets up a visual guide in which identical forms serve to paint many different chords and unlimited sound possibilities. You use the banjo neck to visually realize applied music theory.
See an exploration of the five-note pentatonic scale in the “The Key To Five String Banjo” which will provide a wider understanding of two and three note positions for rapid improvisation.